The present invention is directed to automated analyzers and, more specifically, is directed to a transport delay mechanism for use in a flowing stream reactor that may comprise part of an automated analyzing instrument. Color development in the detection system of an amino acid analyzer has been selected as a representative application.
In some amino acid analysers, a very small or micro chromatographic column is used as a specialized application of a liquid column chromatographic separation technique, utilizing ion exchange resin as the stationary phase and eluting buffers of varying pH and salt concentration as the moving phase. Amino acids contained in a sample are introduced at the top of the column and are separated from each other as they are eluted through the resin bed which comprises the column packing. For amino acid analysis, the method for detecting the amino acids in the effluent stream has been to combine the column effluent with a reagent that is metered into the stream at a flow rate proportional to that of the column eluent. When the reagent combines with the amino acids present in the stream, compounds are formed which, when subjected to further development process, can be detected by specific changes in optical properties such as absorption or fluorescence.
One of the classical detection methods in amino acid analyzer systems is that developed by Spackman and Moore, wherein the reagent used is ninhydrin dissolved in a suitable solvent/buffer solution. Under this process, the column effluent/reagent solution is heated in a reactor to a fixed temperature for a specified period of time. The compound developed as a result of this process will have specific colors, the intensities of which are proportional to the amounts of compounds contained in the flowing stream. The optical density of these compounds is measured at specific wavelengths.
Important to the calibration of the analyzer in terms of its specific sensitivity to detect amino acids is that the fluid/reagent mixture be maintained at a constant elevated temperature for a fixed period of time. It is critical to the stability of the instrument calibration that the two parameters of temperature and exposure time remain constant during the color development process. Classically, this has been accomplished by causing the effluent to pass through a TFE Teflon capillary coil which is normally suspended in a boiling water bath to act as the reactor in the amino acid analyzer system.
The separation techniques employed in early analyzers required several hours to complete a single analysis. In such systems, it became common practice to retain the flowing stream within the reactor for as long as fifteen minutes to complete the color development. Newer techniques have increased the performance of these analyzers to permit the same analyses to be completed in the order of twenty minutes. It has then becomd necessary to provide increased color development in a much shorter period of time. Reference is made to FIG. 1 showing empirical results of studies which relate the optical densities of compounds formed by mixtures of ninhydrin and amino acids as a function of development time and temperature.
It may be noted that maximum sensitivity and improved resolution can be obtained by operating the color development reactor at temperatures significantly above 100.degree. C. However, operation at these elevated temperatures obviates the use of TFE Teflon capillaries immersed in boiling water as has been done in prior analyzers. It then becomes necessary to develop a transport delay system within a heated zone for the flowing liquids which will withstand both the elevated temperature and the corrosive nature of the flowing stream. The elevated temperatures plus the fact that the pH of the solutions alternate between bases and acids increases the corrosive attack by the liquids. Also, at these high temperatures, it is important that the reactor not be damaged by the heat; therefore, the system must provide rapid cooling of the reactor in the event there is some type of catastrophic loss of fluid flow caused by a loss of control in a system.
In most prior art the transport delay for the reactor has been a capillary coil that was usually wrapped in a cylindrical or spiral shape and located in some type of temperature control mechanism. The internal bore and length of this coil were of the proper dimensions to provide a suitable transport time within the heated zone at the flow rates prevailing in the analyzer. However, this type of configuration for the reactor does result in certain limitations with respect to the type of heating that can be used. In most instances, some type of heated bath surrounding the capillary coil has been used. A primary disadvantage with these types of heating systems for reactors is the fact that the cooling capability is not sufficient to provide the protection at higher temperatures against possible boiling of the flowing stream or against damage to the reactor materials themselves.
Because of the numerous disadvantages accruing to the use of a heated reactor in which liquid is used as the heat exchange medium, the design of a reactor using a solid state heat exchange medium has been pursued. In such a development, it soon becomes apparent that the formation of a capillary system into a shape which will permit it to be thermally bonded to a solid state heat exchanger is both difficult and costly. In addition, the cost of fabricating such capillaries from materials which will withstand the gross of nature of the fluids to which they will be subjected in the analyzers is prohibitive for all but the most exotic applications.
Attention is directed to ex,emplary prior systems such as shown in the U.S. Pat. No. 3,806,321 wherein FIG. 5 shows the reaction coil that is used to provide the color development. Another type of reactor is shown in U.S. Pat. No. 4,233,030 wherein the coils are longitudinally doubled back over each other. The U.S. Pat. No. 3,918,907 patent refers to the conventional type of means used to heat the reactor as a reaction coil in an electrically heated boiling bath. Similarly, in U.S. Pat. No. 3,926,800 the color reaction or development coil 23 is placed in a heated bath 24 to develop the color of the column effluent and reagent delivered to the coil.
Attention is directed to copending application entitled "Analytical Instrument Reactor Temperature Regulator", Ser. No. 327,378, filed on Dec. 4, 1981 in the name of Donald E. Stephens and assigned to the assignee of the present patent application. This referenced patent application discloses an alternate method of heating a color development reactor which is in the form of a column.